Detection, Identification And Mitigation Of Ac Faults In Interlinked Ac/Dc Active Distribution Systems

Abstract

he integration of converter-based distributed generators (CBDGs) within AC/DC active distribution systems presents significant challenges in fault detection, identification, and mitigation, primarily due to the limited fault current contributions of these generators. This dissertation investigates and addresses these protection challenges posed by
the behavior of CBDG faults in islanded microgrids by proposing a series of innovative protection schemes designed for both islanded AC microgrids and islanded hybrid AC/DC configurations. For the islanded AC converter-based microgrid, a novel active protection scheme is introduced that employs negative phase sequence (NPS) current injection through a CBDG’s flexible inverter controller. This enhancement in fault detection and isolation eliminates the need for traditional over-current relays, which are ineffective due to the limited fault current. The strategy is implemented by augmenting the traditional droop control mechanism with a positive phase sequence (PPS) voltage
versus NPS voltage droop approach, with differential NPS relays distributed at line ends to ensure accurate fault detection and isolation. Besides, an interharmonic current differential protection scheme is developed for hybrid AC/DC microgrids. This scheme exploits interharmonic frequency variations generated by the ICs to detect and isolate faults in the AC subgrid without relying on high-bandwidth communication. Extensive simulations and real-time hardware-in-the-loop (HIL) experiments validate the performance and reliability of this approach. Additionally, a communication-less protection
approach is proposed for converter-based hybrid microgrids, utilizing virtual impedance fault current limiters (VI-FCLs) to mitigate interharmonic currents and achieve optimal protection coordination (OPC), thereby ensuring reliable fault detection and isolation. The proposed protection scheme’s effectiveness is further validated through a comprehensive real-time HIL experimental setup, demonstrating its practical applicability and reliability. Finally, an advanced fault ride-through (FRT) control scheme is presented for mitigating faults in unbalanced islanded microgrids (UBIMGs). This scheme utilizes an adaptive per-phase unsymmetrical virtual impedance fault limiter (UVIFCL)
to limit fault currents and maintain system stability during and after symmetrical and asymmetrical fault conditions. The proposed protection methods are validated through extensive simulations, demonstrating significant improvements in fault detection, isolation, and mitigation across various fault scenarios. These advancements ensure robust operation and enhanced reliability of AC/DC active distribution systems, particularly addressing the limitations of traditional protection schemes in the presence of limited
fault currents from CBDGs.

Date of Award	13 Dec 2024
Original language	American English
Supervisor	Ehab Fahmy El Sadaany (Supervisor)